There are quite a few posts in the forums regarding thermostat wiring. Hopefully this document can aid you in the replacement of yours

First let’s start off by saying, that there is no official standard for wiring colors, or even terminals for that matter. To be safe it’s always best to refer to your equipment manufactures wiring diagram to see what their terminals are meant for.

Below in the chart, there is a list of each terminal and what it commonly does. As well It will give guidance as to what colors are commonly used. DO NOT ASSUME these are what you have. Verify your wiring first by following it to each piece of equipment. Then you will be able to match the equipment with the terminal.

**-*stat chart.JPG*/-*

More information on the B and X terminals

The wires that appear to cause the greatest amount of confusion are the B and X wires.

Lets look at the B terminal first. The NEMA standard is to use the B terminal for the heating changeover valve however this is not always the case.

Trane and some other manufactures including older Honeywell thermostats use it for the common side of the transformer.

“X” on the other hand is the standard for common, but again like “B” some manufactures use “X” or “X2” as the emergency heat relay.

More information on the B and X terminals

The wires that appear to cause the greatest amount of confusion are the B and X wires.

Lets look at the B terminal first. The NEMA standard is to use the B terminal for the heating changeover valve however this is not always the case.

Trane and some other manufactures including older Honeywell thermostats use it for the common side of the transformer.

“X” on the other hand is the standard for common, but again like “B” some manufactures use “X” or “X2” as the emergency heat relay.